This invention relates to an improvement in diagnostic apparatus utilizing digital imaging techniques, synchronizing and timing circuits for such systems.
The digital X-ray subtraction imaging technique using an imaging intensifier-T.V. chain was developed and first described by Mistretta et al. For example, U.S. Pat. Nos. 4,204,225 and 4,204,226, issued to Charles A. Mistretta, disclose real-time digital X-ray subtraction imaging methods and apparatus which are used in the so-called digital radiography or digital fluoroscopy technology.
These digital X-ray subtraction imaging apparatus include an image intensifier, a T.V. camera whose output is converted into digital format, and an image processor incorporating two digital memories. As disclosed in U.S. Pat. No. 4,204,225, in carrying out the mask mode of this technology a first memory is preferably employed to integrate and store digital mask video signals by digitization of video signals from an image intensifier-T.V. chain during an initial mask time interval utilizing a relatively large number of television fields. A second memory system integrates ongoing digital video signals and provides an output of these integrated video signals, from which the mask video signals from the memory system are subtracted. The resulting digital difference video signals are supplied to a digital-to-analog converter which provides corresponding analog difference video signals for display by a display device. Alternatively, the resulting analog difference video signals may be fed to a multiformat camera for making a selected number of radiographic exposures on a single film.
In the time interval difference mode disclosed in U.S. Pat. No. 4,204,226, a series of difference images is produced by integrating digital video signals over a series of successive time intervals, performing a series of subtractions between the sets of successive integrated video signals stored in the memories to produce a series of digital difference video signals, and converting such digital difference video signals into visibly displayed difference images representing changes in the X-ray image during the successive time intervals.
One advantage of digital radiography apparatus is its capability to perform angiography by means of intravenous injection instead of intra-arterial catheter techniques with their higher risks.
Still another advantage of digital radiography apparatus is its capability to provide improved low contrast detectability, namely, to amplify subtle amounts of contrast media in arteries better than film methods can.
Another advantage of digital radiography apparatus employing pulsed X-rays is that significant loss of spatial resolution due to physiological motion can be prevented because the short radiation pulse for each image results in less loss of detail during fluoroscopy of moving objects.
These digital radiography apparatus include a master timing control circuit which provides synchronization, which circuit is connected to a slave timing control unit for controlling an associated memory unit. The T.V. camera operates in synchronization with the system, via the master timing control which generates sync and blanking signals for the camera video signals.
However, in the prior art systems, the master timing control unit comprises generally a single-phase control clock, so that synchronization errors arise between the sync signals produced by the video camera and a pixel clock via the master timing control circuit, which controls the memory unit for writing in the digital pixel signals. These synchronization errors casue misregistration, and the artifacts from the misregistration appear in the subtraction images. The artifacts from the misregistration are especially harmful when the contrast between picture elements is rather low. It is desirable, therefore, to maintain improved spatial resolution and low contrast detectability by removing or minimizing the synchronization errors.